Microcapsules incorporating a flavor or fragrance compound are useful to provide a controlled release of the contained flavor or fragrance. Such products may be used in the food processing industry, where encapsulated flavor particles may provide a flavor burst upon chewing the food. Such products may also be used in the cosmetic and toiletry industries, where encapsulated fragrance particles may provide a burst of scent upon capsule fracture. The capsule may comprise a shell surrounding a core material in which the flavor or fragrance compound is contained.
Microcapsules may be formed by a coacervation or crosslinking process, in which lipids are coated by tiny droplets of proteins, carbohydrates, or synthetic polymers suspended in water. The process of coacervation is, however, difficult to control and depends on variables such as temperature, pH, agitation of the materials, and the inherent variability introduced by a natural protein or carbohydrate.
In the manufacture of microcapsules containing a flavor or fragrance compound, several features are desirable. It is desirable to produce microcapsules that have strong walls and that do not agglomerate. It is desirable that the compound be readily loaded into an oil microparticle, that is, be readily absorbed into the oil core of the microcapsule. Once absorbed, it is also desirable that the compound be irreversibly retained in the oil core of the microcapsule, that is, be adsorbed into the microcapsule.
The amount of compound that may be encapsulated depends upon several factors including its solubility in water, partition coefficient, molecular weight, water content, volatility, and the ratio of blank capsule to water amounts. Flavors and fragrances may be mixtures of hundreds of components, each of which may widely in these properties. A flavor or fragrance compound that is lipophilic may be readily contained in an oil core of a microcapsule, while a flavor or fragrance compound that is hydrophilic may be less readily contained in an oil core. For example, the flavor compound diacetyl (DA) is about 20% to about 30% water soluble. For diacetyl, typical maximum absorption into an oil is up to only about 55%. A highly water soluble compound such as diacetyl is also more difficult to retain in the oil core once it is loaded.
A compound's solubility in an aqueous phase versus an oil phase is determined by its partition coefficient, abbreviated as K. The partition coefficient of a compound is the ratio of the compound's concentration in one liquid phase to the compound's concentration in another liquid phase. The partition coefficient thus is an inherent property of the compound with two given liquid phases, such as a lipid phase and an aqueous phase, and reflects the compound's distribution at equilibrium between the water phase and the lipid phase. Any means of decreasing the water solubility of a compound will shift the at equilibrium of the compound and thus shift its partitioning between an aqueous phase and a lipid phase. For example, addition of a salt will decrease the water solubility of a compound and will increase its partitioning into the lipid phase. Similarly, crosslinking a protein membrane to strengthen the membrane and physically decrease the amount of water, or physically removing water from the environment, causing capsule wall or membrane shrinking, will decrease the water solubility of a compound and will increase its partitioning into the oil phase.
Flavors or fragrances that are water soluble may interfere with encapsulation of an oil particle. For example, flavor or fragrance compounds that are water soluble cannot be encapsulated using gelatin coacervation. This is because for coacervation to occur, there must be a droplet to coat, and for these water soluble materials, there are no droplets to coat. In addition, the water soluble flavor or fragrance may partition so as to locate the flavor or fragrance compound in an aqueous environment outside the encapsulated oil particle rather than inside the oil particle. If a flavor or fragrance compound is too water soluble, the coacervation process ceases to function due to the colloid becoming either too thick or too thin. A colloid that is too thick has no flow, and thus cannot properly coat the oil surface. A colloid that is too thin is not retained on the oil surface. In the extreme, a water soluble flavor or fragrance compound can totally solubilize the colloid, leaving no wall material to deposit on the oil surface.
Besides water solubility, a flavor or fragrance compound that contains fatty acids affects the pH of a coacervation reaction. If a base is added in an attempt to adjust pH, the fatty salts produced in the reaction impart an undesirable soap taste to a flavor compound. If a flavor or fragrance compound contains water soluble esters, the coacervation temperature is affected and hence the final gelation temperature is altered. While it is therefore desirable to limit compounds that contain either fatty acids or water soluble esters, there is a tradeoff in the potency and profile results for the encapsulated compound. This limits the range of formulations that are able to be effectively encapsulated.
Currently, flavor or fragrance compounds that are difficult to encapsulate are diluted with oils such as vegetable oil or mineral oil. This alters its oil to water partition coefficient, in which the compound attempts to reach an equilibrium between the oil and aqueous phases. The oil serves to reduce the natural water solubility of most compounds and, in many cases, reduces it below the level at which it interferes with coacervation. A flavor or fragrance compound that is highly water soluble, however, does not have this effect. A compound that has a water solubility greater than 25% prefers to partition in an aqueous phase, and a ratio of lipid:water greater than 90% is needed to encapsulate these compounds. The coacervation process, however, is generally limited to about 22% lipid. Thus, this technique is of only limited applicability for water soluble flavor or fragrance compounds.
Several techniques are known in the art for absorbing compounds into a microcapsule, such as cyclodextrin entrapment or silica plating. A drawback of the cyclodextrin entrapment technique is that the binding effect varies widely depending upon the particular flavor or fragrance compound. A drawback of the silica plating technique is that there is no barrier to protect the flavor or fragrance compound from evaporation. Thus, there is a need for an efficient method of absorbing the many types of flavor and fragrance compounds to the desired level of loading in an encapsulated oil. There is also a need for an efficient method of adsorbing flavor and fragrance compounds once they have been encapsulated.